Good explanation, ThisIsTheNameOfTheWebsite, but here are the facts of the matter:

If you meditate into the Spirit World, your body is left in the real world and therefore you cannot bend.

If you enter the Spirit World through a portal, your body is with you and you can bend. 

The Avatar needs her body to enter the Avatar State.

The Spirit Portals are on the same plane as the rest of the Spirit World. 

-Spirit Da7e-